Many of the standard reloading manuals(Lyman comes to mind) will give you the muzzle velocity and muzzle energy in foot pounds ( the amount of energy required to move one pound a distance of one foot) of just about any cartridge, with different loads of powder and length of barrel. Common pistol loads are given with barrel lengths of two to six inches, rifle cartridges from barrels of 18-24 inches. Some loads, like the .44-40, which can be chambered for rifle or pistol, will be shown for both and one will see at once that given the same load, a cartridge fired from a rifle will go faster than the same load from a pistol. With black powder, barrel length difference is even more noticeable.
For comparison a .44 black powder revolver bullet is quite similar to that of the modern .38 special in muzzle energy. The .36 Navy revolver produces figures similar to the later .32 Colt cartridge , actually a somewhat anemic caliber and not an especially good man stopper. In general, it is difficult for black powder to produce the muzzle velocity of more modern smokeless powder so whereas a modern cartridge might drive a 150 grain bullet at 2,000 feet per second with ease, getting a bullet to go beyond 1400 feet per second with black powder is hard, even with a heavy powder load and long barrel and a light bullet. With a very heavy load of black powder you would be shooting yet unignited powder out the muzzle. The black powder muskets of the Revolutionary War through the Civil War derived their killing and knockdown power not from high speed projectiles but from heavy projectiles, 400 -500 grains of lead at modest speeds of about 1,000 feet per second.
On paper such musket loads are not impressive. A .30-30 deer rifle usually produces about 1900 ft. lbs. of energy, a .30-06 or .270 in the 2400 ft. lbs. range while muzzle loading muskets about 1,000 0r 1100 ft. lbs of muzzle energy, BUT, the modern metal jacketed military loads are designed to penetrate and thus much of their energy is expended beyond the target hit (hunting ammo is soft lead, usually rounded or hollow pointed to expand within the target and expend its energy on the target hit). The heavy round balls or minie balls of muskets expand on impact and impart all of their energy to the target, hence my earlier observation that large appliances hit with modern high speed bullets just get neat holes put through them while the musket balls actually made huge holes and moved the targets. The professional buffalo hunters with their Sharps in .45-90 caliber had no trouble bringing down 1500 pound buffalo with single shots. Other variables are involved, but, the bottom line on this is that large caliber pure lead slugs at modest muzzle speeds are as lethal as anything being used today at distances of less than about 150 yards, in my opinion.